Add resizeBuffer

[sorki]

For scenarios where input can change over time, this adds resizeBuffer function to produce a new buffer with specified length.

This causes transparent reallocation of OpenGL buffer, producing new empty buffer of right size.

[sorki]

Thinking about this, it's not ideal fit for this library since it voids the type-safetiness goal.

Would you accept this if renamed to unsafeResizeBuffer?

Or alternatively, I can carry this downstream but that requires re-exporting couple of internals:

• bufName, bufElementSize from Graphics.GPipe(.Internal).Buffer
• liftNonWinContextIO from Graphics.GPipe(.Internal).Context.

[tobbebex]

Closing this old PR since its not safe indeed